Since I can't resist the challenge of solving math problems, I figure that an undrilled octagonal barrel blank 13/16 across the flats that's 32 inches long should weigh about 4.87 pounds, figuring steel having a density of .278 pounds per cubic inch.
Drilling a .32 diameter hole through it should remove .715 pounds of steel.
Drilling a .36 diameter hole through it should remove .906 pounds of steel.
Drilling a .40 diameter hole through it should remove 1.12 pounds of steel.